From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from cn.fujitsu.com ([59.151.112.132]:13031 "EHLO heian.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-FAIL) by vger.kernel.org with ESMTP id S1751149AbaJ0Ie6 convert rfc822-to-8bit (ORCPT ); Mon, 27 Oct 2014 04:34:58 -0400 Message-ID: <544E040A.3090407@cn.fujitsu.com> Date: Mon, 27 Oct 2014 16:36:26 +0800 From: Qu Wenruo MIME-Version: 1.0 To: CC: Subject: Re: [PATCH] btrfs: Enhance btrfs chunk allocation algorithm to reduce ENOSPC caused by unbalanced data/metadata allocation. References: <1414031871-10859-1-git-send-email-quwenruo@cn.fujitsu.com> <20141024110624.GB32526@localhost.localdomain> <544D8F44.8050706@cn.fujitsu.com> <20141027081456.GD27271@localhost.localdomain> In-Reply-To: <20141027081456.GD27271@localhost.localdomain> Content-Type: text/plain; charset="utf-8"; format=flowed Sender: linux-btrfs-owner@vger.kernel.org List-ID: -------- Original Message -------- Subject: Re: [PATCH] btrfs: Enhance btrfs chunk allocation algorithm to reduce ENOSPC caused by unbalanced data/metadata allocation. From: Liu Bo To: Qu Wenruo Date: 2014年10月27日 16:14 > On Mon, Oct 27, 2014 at 08:18:12AM +0800, Qu Wenruo wrote: >> -------- Original Message -------- >> Subject: Re: [PATCH] btrfs: Enhance btrfs chunk allocation algorithm >> to reduce ENOSPC caused by unbalanced data/metadata allocation. >> From: Liu Bo >> To: Qu Wenruo >> Date: 2014年10月24日 19:06 >>> On Thu, Oct 23, 2014 at 10:37:51AM +0800, Qu Wenruo wrote: >>>> When btrfs allocate a chunk, it will try to alloc up to 1G for data and >>>> 256M for metadata, or 10% of all the writeable space if there is enough >>> 10G for data, >>> if (type & BTRFS_BLOCK_GROUP_DATA) { >>> max_stripe_size = 1024 * 1024 * 1024; >>> max_chunk_size = 10 * max_stripe_size; >> Oh, sorry, 10G is right. >> >> Any other comments? >> >> Thanks, >> Qu >> >> >>> ... >>> >>> thanks, >>> -liubo >>> >>>> space for the stripe on device. >>>> >>>> However, when we run out of space, this allocation may cause unbalanced >>>> chunk allocation. >>>> For example, there are only 1G unallocated space, and request for >>>> allocate DATA chunk is sent, and all the space will be allocated as data >>>> chunk, making later metadata chunk alloc request unable to handle, which >>>> will cause ENOSPC. >>>> This is the one of the common complains from end users about why ENOSPC >>>> happens but there is still available space. > Okay, I don't think this is the common case, AFAIK, the most ENOSPC is caused > by our runtime worst case metadata reservation problem. > > btrfs has been inclined to create a fairly large metadata chunk (1G) in its > initial mkfs stage and 256M metadata chunk is also a very large one. > > As of your below example, yes, we don't have space for metadata > allocation, but do we really need to allocate a new one? > > Or am I missing something? > > thanks, > -liubo Yes that's true this is not the common cause, but at least this patch may make the percentage of 'df' command reach as close to 100% as possible before hitting ENOSPC under normal operations. (If not using balance) And some case like the following mail may be improved by the patch: https://www.mail-archive.com/linux-btrfs@vger.kernel.org/msg36097.html I understand that most of the cases that a lot of free data space and no metadata space is caused by create and then delete large files, but if the last giga bytes can be allocated more carefully, at least the available bytes of 'df' command should be reduced before hit ENOSPC. How do you think about it? Thanks, Qu > >>>> This patch will try not to alloc chunk which is more than half of the >>>> unallocated space, making the last space more balanced at a small cost >>>> of more fragmented chunk at the last 1G. >>>> >>>> Some easy example: >>>> Preallocate 17.5G on a 20G empty btrfs fs: >>>> [Before] >>>> # btrfs fi show /mnt/test >>>> Label: none uuid: da8741b1-5d47-4245-9e94-bfccea34e91e >>>> Total devices 1 FS bytes used 17.50GiB >>>> devid 1 size 20.00GiB used 20.00GiB path /dev/sdb >>>> All space is allocated. No space later metadata space. >>>> >>>> [After] >>>> # btrfs fi show /mnt/test >>>> Label: none uuid: e6935aeb-a232-4140-84f9-80aab1f23d56 >>>> Total devices 1 FS bytes used 17.50GiB >>>> devid 1 size 20.00GiB used 19.77GiB path /dev/sdb >>>> About 230M is still available for later metadata allocation. >>>> >>>> Signed-off-by: Qu Wenruo >>>> --- >>>> fs/btrfs/volumes.c | 18 ++++++++++++++++++ >>>> 1 file changed, 18 insertions(+) >>>> >>>> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c >>>> index d47289c..fa8de79 100644 >>>> --- a/fs/btrfs/volumes.c >>>> +++ b/fs/btrfs/volumes.c >>>> @@ -4240,6 +4240,7 @@ static int __btrfs_alloc_chunk(struct btrfs_trans_handle *trans, >>>> int ret; >>>> u64 max_stripe_size; >>>> u64 max_chunk_size; >>>> + u64 total_avail_space = 0; >>>> u64 stripe_size; >>>> u64 num_bytes; >>>> u64 raid_stripe_len = BTRFS_STRIPE_LEN; >>>> @@ -4352,10 +4353,27 @@ static int __btrfs_alloc_chunk(struct btrfs_trans_handle *trans, >>>> devices_info[ndevs].max_avail = max_avail; >>>> devices_info[ndevs].total_avail = total_avail; >>>> devices_info[ndevs].dev = device; >>>> + total_avail_space += total_avail; >>>> ++ndevs; >>>> } >>>> /* >>>> + * Try not to occupy more than half of the unallocated space. >>>> + * When run short of space and alloc all the space to >>>> + * data/metadata will cause ENOSPC to be triggered more easily. >>>> + * >>>> + * And since the minimum chunk size is 16M, the half-half will cause >>>> + * 16M allocated from 20M available space and reset 4M will not be >>>> + * used ever. In that case(16~32M), allocate all directly. >>>> + */ >>>> + if (total_avail_space < 32 * 1024 * 1024 && >>>> + total_avail_space > 16 * 1024 * 1024) >>>> + max_chunk_size = total_avail_space; >>>> + else >>>> + max_chunk_size = min(total_avail_space / 2, max_chunk_size); >>>> + max_chunk_size = min(total_avail_space / 2, max_chunk_size); >>>> + >>>> + /* >>>> * now sort the devices by hole size / available space >>>> */ >>>> sort(devices_info, ndevs, sizeof(struct btrfs_device_info), >>>> -- >>>> 2.1.2 >>>> >>>> -- >>>> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in >>>> the body of a message to majordomo@vger.kernel.org >>>> More majordomo info at http://vger.kernel.org/majordomo-info.html